1. Field of the Invention
The present invention relates to an image processing apparatus, a printing apparatus and an image processing method, and particularly to color separation in which a particular color recording material is used at a low lightness portion in a color reproduction range by a printing apparatus.
2. Description of the Related Art
An image forming apparatus that forms image information (including characters, symbols and the like) on a print medium is known as a printer, a copier, and a facsimile machine or an output device of information processed by a computer or a word processor. Such an image processing apparatus performs the printing based upon image information with use of a printing mechanism, on the print medium such as papers or plastic thin sheets (OHP sheet). Examples representative of these printing systems include an ink jet system, a wire dot system, a thermo-sensitive system, a thermal-transfer system, an electro-photographic system using a laser beam and the like, and the like.
Among these printing systems described above, the ink jet system ejects ink from a print head in the form of drop to be landed to the print medium for forming dots to generate an image thereon. An inkjet printer using this system is widely used recently as an output device for an information processing equipment such as a computer or the like.
In the ink jet printer, dyes or pigments are used as coloring materials contained in the ink as recording materials. Basic colors of cyan (C), magenta (M) and yellow (Y) as three primary colors of the subtractive color mixing are used as ink colors by the coloring materials. Also, a system using four colors containing further black (K) is generally used. This system can realize reduction in an ink applying amount to the print medium or the density improvement, by further using black. Further, in a system for a high image quality of a photographic image, the ink of light cyan (LC) and light magenta (LM), which have lower concentration than that of cyan and magenta inks, may be used as quasi-basic color ink in addition to the ink of the above four colors. This allows the image quality improvement such as reducing a granular feeling on a highlight part of the printed image. Also, print systems further using ink of gray (Gry) and light gray (light Gry) are known.
In the meantime, a demand for higher image quality is recently increasing with wide use of a color printer. However, the above stated basic colors and quasi-basic colors have the limitation in reproducing color and it is hard to reproduce a relative bright color therewith. Therefore, particular colors such as red (R), orange (O), green (G), blue (B), violet (V) and the like in addition to the basic colors and the quasi-basic colors are used to realize color reproduction with high chroma which is impossible to be reproduced only with the basic color and the quasi-basic color.
Japanese Patent Laid-Open No. 10-112809 (1998) discloses that with regard to basic color recording materials produced by color separation, color separation in a system of using particular colors is made by exchanging two of the basic colors for the particular color obtained by mixing the two basic color recording materials. More specifically, Japanese Patent Laid-Open No. 10-112809 discloses the color separation for broadening a color reproduction region by using particular color O ink in addition to inks of the basic colors M and Y, with regard to the hue between respective hues of M and Y as shown in FIG. 1.
Further, recently, color printers with high photographic image quality oriented for photographers or high level amateurs of photos, among color printers using particular colors, have been widely used. In such printer with high photographic image quality, a delicate color reproduction is required even in a gamut to which attention is not paid in a conventional color printer. One example is a delicate expression of a gradation at a dark portion (low lightness portion) in a color reproduction region by a printer. For realizing the delicate gradation expression at the dark portion, the chroma in the dark portion of the color reproduction region by the printer becomes important.
FIG. 2 is a diagram showing a color reproduction region by a printer on a C* L* plane. For example, a color reproduction region in a case where the chroma at the dark portion of printer color reproduction region is low, has an out line that is shown in color reproduction line a1, b1, b2 or b3. On the other hand, a color reproduction region in a case where the chroma at the dark portion of printer color reproduction region is high is shown in color reproduction line b4 or b5.
FIGS. 3 and 4 show views showing how a difference in chroma at the dark portion of color reproduction region influences an image quality and show a photographic image of green ivy. FIGS. 3 and 4 correspond to respective printing based on the same data of a photographic image, in a case of using a color reproduction region where the chroma at the dark portion is low as the color reproduction line b1 shown in FIG. 2 and in a case of using a color reproduction region where the chroma at the dark portion is high as the color reproduction line b4 shown in FIG. 2. The respective printed images are color images in which the lightness is nearly equal and the chroma is different. In the present specification, for explaining the image with a binary image of white and black, FIGS. 3 and 4 are expressed as binary images of white and black which are subjected to a process of extracting the chroma C* only.
As described above, FIG. 3 shows an image of green ivy by the color reproduction region where the chroma at the dark portion is low, and FIG. 4 shows an image of green ivy by the color reproduction region where the chroma at the dark portion is high. In these figures, the portion shown as black is a low chroma region and the portion shown as white is a high chroma region. Here, a comparison of an image (attention part P1) by the color reproduction region in which the chroma at the dark portion is low with an image (attention part P2) by the color reproduction region where the chroma at the dark portion is high is as follows. In the attention part P1 in FIG. 3, the shade portion of an ivy leaf becomes black wholly and the detail can not be determined. On the other hand, in the attention part P2 in FIG. 4, even the shade portion of an ivy leaf is expressed in white and with high chroma and the detail can be determined. Here, the image of the green ivy is exemplified, but when an image like a leaf of an acicular tree is used, the number of leaves which can be determined changes depending on the chroma at the dark portion. In addition, the above result is applied to not only an image in green but also an image of hair of humans or animals in a different hue.
As described above, by performing the color reproduction with higher chroma even at the dark portion in the printer gamut, more delicate color reproduction (gradation value) can be made.
As the conventional art where attention is paid even to the color reproduction at the dark portion, there is known a color separation with respect to basic colors which is disclosed in Japanese Patent Laid-Open No. 2002-10096. Japanese Patent Laid-Open No. 2002-10096 discloses the art which sets the amount of black ink so that the maximum gamut is obtained within the ejection amount (coverage) limit, in order to broaden the gamut of the color reproduction region at the dark portion in a printer of four colors of C, M, Y and K.
In addition, as the conventional arts where attention is paid to color reproduction at the dark portion in a system using a particular color, there are known arts disclosed in Japanese Patent Laid-Open Nos. 2003-11432, 2005-205812, and 2005-217987. Japanese Patent Laid-Open Nos. 2003-11432, 2005-205812, and 2005-217987 disclose a system using particular color (G) ink realizing high lightness and high chroma, as shown in FIG. 5. More specifically, when reproducing a secondary color, a particular color is positively used in the high lightness region where reproduction region broadening is effectively made by the particular color. Then, since use of color mixture of basic color inks may broaden the color reproduction region more than use of the particular color at the low lightness portion (dark portion), the particular color ink is less used and the chroma is expressed with color mixing of the basic colors. FIG. 6 is a diagram showing one example of this color separation. As shown in the figure, according to the color separation disclosed by Japanese Patent Laid-Open No. 2003-11432 or the like, the color reproduction in the light region is made using the particular color (G) ink in FIG. 5 and the color reproduction in the dark region is made using C ink and Y ink mainly.
Further, Japanese Patent Laid-Open Nos. 2005-059361 and 2005-059360 describe color separation that uses a particular color as a complementary color component for a basic color at a low lightness portion of the hue opposite the hue of the particular color so as to broaden a color reproduction region at a dark portion, in a system using the particular color for realizing high lightness and high chroma. FIG. 7 is a diagram showing one example of this color separation. As shown in the figure, the color reproduction in a primary color Cyan-Black having the hue opposite the hue of the particular color Red is realized by using the particular color R ink as a complementary color component for basic color C ink. In addition, these patent documents, as shown in FIG. 7, further describe that a secondary color Red-Black is reproduced using the particular color R ink, as well as basic color C ink and quasi-basic color K ink as complementary color components.
The color separation of conventional arts described above is, however, not sufficient in view of increasing the chroma at a low lightness portion.
Japanese Patent Laid-Open No. 10-112809 does not disclose any color separation that realizes broadening the reproduction region at the dark portion in a color reproduction region of a printer. Japanese Patent Laid-Open No. 2002-10096 discloses the color separation where attention is paid to broadening the reproduction region at the dark portion, but uses only basic colors without using particular colors. Therefore, an improvement of sufficient chroma at the dark portion can not be expected. In addition, Japanese Patent Laid-Open Nos. 2003-11432, 2005-205812 and 2005-217987 basically relate to the color separation in less use of the particular color at the dark portion. Therefore, likewise, an improvement of sufficient chroma at the dark portion can not be expected.
On the other hand, Japanese Patent Laid-Open Nos. 2005-059361 and 2005-059360 disclose that as described above, the color reproduction at the back portion of secondary color Red-Black is made using particular color R ink having the hue similar to that of the secondary color and basic colors of M and Y ink, and also using K and C ink as complementary colors. According to these color separations, it is possible to broaden the color reproduction region at the dark portion (low lightness portion).
However, since the color separation uses the basic color and the quasi-basic color as the complementary color components, there is a case where these complementary colors do not become idealized complementary colors for the particular color in terms of a reflection spectrum. As a result, the color reproduction where the lightness is sufficiently lowered and at the same time, reduction of the chroma is prevented may not be realized. Thus, it may not be possible to realize high chroma at the dark portion.